This invention relates to a method of utilizing a composite powder of coal combustion fly ash containing desulfurization residue to amend agricultural soil. More particularly, this invention relates to the conduct of such a method in a manner that not only prevents the escape of fugitive dust from the powder but also effectively manages any cementitious tendencies of the powder.
The combustion of coal to generate electrical power at utility plants has become more and more widespread and produces huge quantities of fly ash particles as well as noxious sulfur-containing gases that would escape to the atmosphere if not specially removed and collected. Removal or trapping of the sulfur gases is accomplished by desulfurization techniques that rely on reacting the sulfur of the gases with calcium compounds to form an easily collectable solidified residue containing, for example, calcium sulfate. This sulfur-containing residue and any excess calcium compound are collected with the fly ash from the flue gases of combustion as a composite powder. Most of this powder (i.e., fly ash containing desulfurization residue) is presently being disposed of in landfills, but landfilling the considerable amounts generated (e.g., 250,000 U.S. tons or more per year from a single electrical power plant) is burdensome and expensive and considered objectionable.
The calcium-enriched fly ash composite powder has been shown in pot studies or on small test plots to be beneficial as an agricultural soil amendment for raising the pH of acidic soils and for adding beneficial plant nutrients. However, insofar as is known, this ash powder has not seen any significant agricultural utilization as a soil amendment due to the difficulties associated with handling and land applying the tonnage quantities necessary for a meaningful agricultural field utilization of it.
When applied in a dry condition to the surface of a field of agricultural soil, this fly ash powder has proven difficult to manage because the powder's fine, dusty character makes it vulnerable to the problem of "fugitive dust." Fugitive dust is small ash powder particles that are easily blown about by the wind and thus "escape" from powder applied on the soil's surface. The wind-driven fugitive dust is a special problem because it may travel to untreated fields or places not suitable for the ash powder, such as in surface water or residential areas. It also can cause operator discomfort and can clog filtering systems for mechanical equipment.
Injecting the dry ash powder below the surface of the soil has been considered to avoid the dust problem. However, injection would concentrate the ash powder in a relatively small area or "band" within the soil. This is highly significant because some constituents of the ash powder (such as, for example, boron) that are relatively innocuous and even beneficial to plants at lower concentration levels in the soil may reach a toxic level when they are concentrated in a localized area of the soil. Also, the soil pH-raising ability of the powder would be limited to only a small area around the band.
A treatment technique that has been considered is to apply the dry ash powder to the surface of the soil and subsequently turn the ash-covered soil surface over using a plow (e.g., a moldboard plow) to bury the powder. This technique is unsatisfactory because the powder is still vulnerable to fugitive dust during application and until the powder is plowed beneath the surface. Also, if the plowing is done improperly, the powder can end up being concentrated into a strata lying below the soil's surface, which creates problems of potential toxicity and isolated pH-raising effect similar to those arising from injection application.
It has been widely believed that the addition of water to the high calcium fly ash powder to form a composition for application to agricultural soil would create difficulties even more serious than those of dry application because of the widespread tendency of such ashes to exhibit a latently cementitious character. The latently cementitious property has been known to cause hardening or curing of the wetted powder into a solid mass. It is the very property relied upon in Webster's U.S. Pat. No. 4,354,876 in forming compacted solid structural landfills. A major concern has been that the addition of water to the powder would cause it to harden or cure to a set up condition during land spreading operations, and would therefore clog and damage the equipment employed in land spreading the tonnage quantities required for effective amendment of agricultural fields.
These vexing problems have heretofore prevented any significant agricultural field utilization of the composite powder of fly ash containing desulfurization residue, despite the fact that usefulness of the powder as a potential soil amendment has been known for a long time, even decades.